This invention relates to improvements in magnetically actuated reed switches, and particularly to those using permanent magnet biasing for adjusting the sensitivity of a reed switch to changes in magnetic flux density.
A reed switch comprises a tubular glass envelope containing electrical contacts attached to magnetic material, that is, material which becomes magnetized in the presence of a magnetic field, at least one such contact being located at a movable end of a flexible elongate blade, or "reed", of magnetic material. Such a reed switch is operated by increasing the magnetic flux density in the vicinity of the magnetic portions of the switch, thereby magnetizing those portions, which causes the reed to be drawn to another magnetic element of the switch. When the magnetic flux density is reduced, the reed is released and the reed switch resumes its normal condition. In the various forms of reed switches the electrical contacts may move under the influence of a magnetic field either to complete or interrupt a circuit as a single-pole-single-throw switch, or the reed-carried contact may move from a closed position with respect to one fixed contact to a closed position with respect to another fixed contact as a single-pole-double-throw switch.
In security systems magnetic reed switches are often used in conjunction with an actuating permanent magnet which, when close enough to the reed switch, actuates the switch. For example, a switch and its associated electrical conductors, leading to a monitoring control unit and alarm device, may be mounted in or on the frame surrounding a doorway or window opening, with the actuating magnet located in or on the door or window itself, so that location of the door or window in a predetermined position actuates the reed switch, and movement from that position releases the reed switch, producing a signal received by the monitoring device. However, the sensitivity of a reed switch to such actuation is limited, so such an application requires the use of either a strong actuating magnet, or an installation providing a very small gap between the actuating magnet and the sensing reed switch. Moreover, careful alignment of the switch and the actuating magnet is required.
More particularly, in environments containing magnetic materials such as steel fire doors, low sensitivity of the reed switch has caused the reliability of such security switches to be less than desired, since a slight movement of the actuating magnet could allow the magnetically held components to be released. For example, such switches installed to monitor a door can produce false alarms if wind gusts cause the door to shift slightly, and switches monitoring overhead or sliding doors can produce an erroneous indication of the position of such doors because of minor misalignment in their tracks.
The use of larger actuating magnets mounted on doors and windows allows these insensitive reed switches to remain actuated despite small movements of doors and windows, reducing the number of false alarms, but making such a magnetic switch more easily detected and located by a magnetometer, and thus less secure from tampering. Additionally, the material used in producing the actuating magnets is not inexpensive, and larger magnets appreciably increase the cost of the devices. Also, the smaller the actuating magnet, the more easily its installation may be concealed.
As well as in security systems, magnetically actuated switches are useful for machine control applications, and the low sensitivity of conventional reed switches is also often of concern in that environment.
One means of increasing the sensitivity of such a reed switch is to place a permanent magnet near the switch to bias the reed by providing part of the magnetic flux density necessary for actuation. Permanent magnet bias means for controlling the sensitivity of reed switches have previously been disclosed, for example by varying the distance or angular relationship between the magnet and the longitudinal axis of the reed of the switch, as shown in Nicholls U.S. Pat. No. 3,974,469, and by varying the location of the reed along an imaginary axis parallel to the axis of polarity of the biasing magnet, as shown by Tann U.S. Pat. No. 3,305,805. However, the device of the Nicholls patent requires either a threaded hole in the biasing magnet in order to vary its distance, or a disc-shaped rotatable magnet and socket to vary its angular relationship, both of which are somewhat expensive and complicated means of attachment, and Tann shows no apparatus for holding the axis of polarity of a biasing magnet parallel to the reed of a reed switch. Also, while Tann shows a means of adjustment of the position of a reed switch relative to a magnet, only discrete, not continuous, variation of position is provided. Accordingly, there is a need for a reed switch having an inexpensive and simple means of attaching a small biasing permanent magnet to the reed switch in a continuously adjustable position parallel thereto for producing a desired level of sensitivity.